It's a little unorthodox, but not racist. I'm sure if you say that you aren't comfortable living somewhere where everyone is part of the same religion, they would understand. I wouldn't feel comfortable living with a house full of Jehovah's Witnesses.

I wouldn't say you're being racist but please don't assume that all Indians are Hindus and share ethical beliefs. Would you say all Britishers are Christians and share ethical beliefs? If your friend knows the other flatmates and already lives there it may be an idea to socialise with them and visit the house to see what it's like?

Tricky one this.
I suppose it comes down to personal choice.
But people might be quick to brand it as being a racist.
My gut is saying that you should do what you want and not let it worry you.
After all you are not dissadvantaging them, which is sometimes what being racist is about.

I might be horribly wrong though and anyone who wishes to correct me is welcome.